Where does “三時” come from?

三時 (Japanese) comes from Japanese 時, from Old Japanese 時, from Old Japanese しだ — a time, circumstance, occasion.

三時 (Japanese): three o'clock

Definitions

三時 traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	時	time; an event, case, occasion, moment; a...
2	Old Japanese	時	a time, moment (in time), occasion
3	Old Japanese	しだ	a time, circumstance, occasion

Step	Language	Word	Meaning
1	Japanese	三	three, 3; third; three
2	Middle Chinese	さん	—